[By what is the entry of Maghrib known?]

Answer: The entry of Maghrib is known by:
1. the appearance of a night star—known to be “night” by being seen in the northern half of the sky;
2. the emergence of darkness in the places of the sun’s rising;
3. the light of the moon appearing—i.e., a shine on its face.
There is no disagreement that the first time of Maghrib is immediately after sunset; but we do not ascertain that it has set except by such signs—for it may be concealed by a mountain, a height, or clouds. In Jābir’s ḥadīth on the Prophet’s Ḥajj (May Allah bless him and his family and grant them peace), it is reported that he (May Allah bless him and his family and grant them peace) departed from ʿArafah when the sun had set and its yellow glow had diminished a little; thus he (May Allah bless him and his family and grant them peace) did not depart at the very moment of sunset, but delayed until the yellow glow had slightly faded. From this it is inferred that what is meant by sunset is the setting of the sun and its rays, not its mere concealment behind a mountain and the like.
The preferred time of Maghrib ends with the disappearance of the red twilight. Some scholars have estimated it as the time sufficient for Maghrib, its regular Sunnah, the two rakʿahs of Maghrib, and the two rakʿahs of al-Furqān.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1